The need to reduce the cost and size of electronic equipment has led to a continuing need for smaller filter elements. Consumer electronics such as cellular telephones and miniature radios place severe limitations on both the size and cost of the components contained therein. Many such devices utilize filters that must be tuned to precise frequencies. Hence, there has been a continuing effort to provide inexpensive, compact filter units.
One class of filter element that has the potential for meeting these needs is constructed from acoustic resonators. These devices use bulk longitudinal acoustic waves in thin film piezoelectric (PZ) material. In one simple configuration, a layer of PZ material is sandwiched between two metal electrodes. The sandwich structure is suspended in air by supporting it around the perimeter. When an electric field is created between the two electrodes via an impressed voltage, the PZ material converts some of the electrical energy into mechanical energy in the form of sound waves. The sound waves can propagate longitudinally in the same direction as the electric field and reflect off the electrode/air interface, or in a direction transverse to the electric field and reflect off the various discontinuities at the edges of the electrodes or the structure.
The device is a mechanical resonator which can be electronically coupled; hence, the device can act as a filter. For a given phase velocity of sound in the material, the mechanical resonant frequency is that for which the half wavelength of the sound wave propagating longitudinally in the device is equal to the total thickness of the device. Since the velocity of sound is four orders of magnitude smaller than the velocity of light, the resulting resonator can be quite compact. Resonators for applications in the GHz range may be constructed with physical dimensions less than 100 microns in diameter and a few microns in thickness.
At the heart of Thin Film Bulk Acoustic Resonators (FBARs) and Stacked Thin Film Bulk Wave Acoustic Resonators and Filters (SBARs) is a thin sputtered piezoelectric film having a thickness on the order of one to two microns. Electrodes on top and bottom sandwich the piezoelectric film to provide an electric field through the piezoelectric material. The piezoelectric material, in turn, converts a fraction of the electric field into a mechanical field. An FBAR is a single layer of PZ material and acts as an absorption filter. An SBAR is constructed by stacking two or more layers of PZ material with electrodes between the layers and on the top and bottom of the stack. An SBAR is typically used for a transmission filter.
To simplify the following discussion, the present invention will be explained in terms of an FBAR; however, it will be apparent from the discussion that the teachings of the present invention are also applicable to SBARs as well. The portion of the PZ film included between the electrode overlap forms an acoustical cavity. The primary oscillatory mode of this cavity is that in which sound waves, either compression, shear, or of plate wave type, propagate in a direction perpendicular to the plane of the electrodes. Unfortunately, there are other oscillatory modes that can be excited. These modes correspond to sound waves travelling parallel to the plane of the electrodes and bouncing off the walls of the cavity or the discontinuity at the edge of the electrode layers. The primary frequency of these modes is much lower than the primary mode, however, the higher harmonics of these transverse modes can appear within the frequency band of the primary mode. These harmonics lead to a number of "spikes" or other irregularities in the absorption spectrum of the FBAR. While the total energy absorbed by these spikes is relatively small, the irregularities can still cause problems in circuits employing filters of the FBAR and SBAR variety.
Broadly, it is the object of the present invention to provide an improved bulk acoustical resonator.
It is a further object of the present invention to provide a bulk acoustical resonator that has an absorption and/or transmission spectrum that does not include irregularities generated by transverse resonant modes.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.